The number of Nazi concentration camps and ghettos that existed during Hitler's reign of terror between 1933 to 1945 actually amounted to a shocking 42,500, six times more than the previous estimate of 7,000.

Researchers from the US Holocaust Memorial Museum catalogued some 42,500
Nazi ghettos and camps throughout Europe, spanning German-controlled areas from France to Russia and Germany itself.

The total is far higher than most historians had previously estimated, 'The New York Times' reported.

The Museum team also created maps of the sites, which were scattered across Europe, and which imprisoned or killed between 15 and 20 million people.

Geoffrey Megargee and Martin Dean, the lead editors of the project, have compiled the thousands of sites in a multi-volume encyclopedia that is being published by the Holocaust Museum.

The documented camps include thousands of forced labour camps, where prisoners manufactured war supplies; prisoner-of-war camps, sites euphemistically named "care" centers, where pregnant women were forced to have abortions or their babies were killed after birth.

"The numbers are so much higher than what we originally thought," Hartmut Berghoff, director of the institute, said.

"We knew before how horrible life in the camps and ghettos was but the numbers are unbelievable," the paper quoted him as saying.

Over the years, many scholars have worked to uncover the lost or unknown victims of the Holocaust, and some have insisted the death toll is higher than what the textbooks say.